Optical amplifiers using cladding-pumped fibers (CPFs) are useful for amplifying optical signals to powers of several watts or more. In a typical arrangement for optical amplification, the refractive index of the CPF exhibits a radial profile that defines a pair of concentric waveguiding regions.
Light at the signal wavelength, which is to be amplified, is confined to an inner waveguide defined by the core and inner cladding of the fiber. The inner cladding is defined in the glass fiber and juxtaposed to the core. The core is doped with an active species such as erbium or another rare-earth element which is effective for providing amplification when appropriately pumped.
Pump radiation is confined to an outer waveguide defined by the inner cladding mentioned above, together with an outer cladding. The outer cladding is typically a low-refractive-index polymer layer overcoated onto the glass inner cladding mentioned above. In alternative arrangements, the outer cladding may be a further layer of glass, or even air or another ambient atmosphere, or the core and inner cladding could be composed of another material such as a polymer. The fiber dimensions and the index profile are typically arranged to provide a relatively high numerical aperture (NA)—typically about 0.45—as well as a relatively large diameter for the outer waveguide. These features promote the efficient coupling of pump light into the outer waveguide from semiconductor diode pump lasers.
It is often desirable to pool pump light from multiple laser diodes. A tapered fiber bundle (TFB) is one arrangement useful for this purpose. The TFB is a close-packed array of optical fibers that are fused together and and tapered down to a diameter that matches the inner cladding diameter of the CPF. TFBs are described, for example, in U.S. Pat. No. 5,864,644, commonly assigned herewith.
In a bundle of, e.g., seven fibers, six large-core multimode (MM) fibers are typically packed around a central fiber that is single mode or at most supports a few optical modes, and that is used to guide signal light through the bundle. The input (i.e., the unfused) end of each of the MM fibers is typically fusion spliced to the pigtail from a respective laser diode. Collectively, the MM fibers combine the pump light from their respective laser diodes and deliver it to the output (fused) end of the bundle, and from there into the CPF.
In typical arrangements, the input (unfused) end of the central fiber is fusion spliced to a standard single-mode fiber, or other suitable fiber, that delivers the input signal.
The output (fused) end of the bundle is typically fusion spliced to an output fiber pigtail structured to have concentric inner and outer waveguides. At the output end of the bundle, the central fiber of the bundle also effectively has an inner waveguide, which is matched to the inner waveguide of the pigtail. The outer waveguide of the pigtail is matched to the outer diameter of the entire bundle (at the fused end), and also matched to the outer waveguide of the CPF.
In practice, TFB-CPF assemblies may be arranged such that relative to the signal light, the pump light is co-propagating, counter-propagating, or both.
One problem that is known to occur in amplifiers of the CPF type is uncontrolled lasing. When uncontrolled lasing occurs, it can disrupt the output signal from the amplifier, and can even damage the amplifier or other components. Uncontrolled lasing can be initiated by reflections at the signal wavelength, or at other wavelengths, that reenter the CPF.
It is desirable to find ways to mitigate uncontrolled lasing that may occur when cladding-pumped fibers are spliced to tapered fiber bundles.